1. Field of the Invention
The present disclosure relates to a transmitter and a transmitting method thereof in a wireless communication system, and more particularly, to a transmitter of a wireless communication system, which can transmit radio frequency (RF) signals using an outphasing scheme of converting one analog intermediate frequency non-constant envelope modulated signal (IF NC-EMS) into two analog IF C-EMSs, and a transmitting method thereof.
2. Description of the Related Art
Recently, many studies have been conducted on multi-band and low-power one-chip solutions in wireless communication systems. Because RF transmitters are essential to wireless communication systems, there is an increasing demand for optimization requirement in association with multi-band and low-power one-chip solutions.
A variety of modulation schemes are used in wireless communication systems. As a data rate is increasing, a modulation scheme becomes more complicated. To support a high data rate, most of modulation schemes store information in both amplitude and phase. In this way, such modulation schemes can increase the frequency bandwidth efficiency and the data rate. At this point, the modulated signal is a non-constant envelope modulated signal (NC-EMS). A transmitter that processes an NC-EMS requires a back-off margin of a peak-to-average ratio (PAR) higher than a transmitter that processes a constant envelope modulated signal (C-EMS). Therefore, a power amplifier having a higher linearity is needed, thus increasing the transmission power consumption of the transmitter.
Meanwhile, a super-heterodyne system is widely used as a transmission scheme in wireless communication systems. Examples of the super-heterodyne transmitter include a polar transmitter and a combined analog locked loop universal modulation (CALLUM) transmitter.
FIG. 1 is a block diagram of a conventional polar transmitter.
The conventional polar transmitter of FIG. 1 has difficulty in sorting phases/amplitudes of desired signals because of a frequency-dependant delay between an envelope and a phase. Further, the conventional polar transmitter must overcome a limited frequency response of a DC/DC converter and a limited linearity of an envelope magnitude. Moreover, a general baseband modem cannot form an entire transmission path. Thus, a polar-dedicated baseband modem must be provided for forming the entire transmission path.
FIG. 2 is a block diagram of a conventional CALLUM transmitter.
In the conventional CALLUM transmitter of FIG. 2, errors occurring in a down-conversion again appear in a final output. The hardware architecture of a feedback system is complicated and the competitiveness in high-speed broadband operation is weak.
The conventional transmitters perform a frequency up-conversion using one or more intermediate frequencies and complicated hardware is used in the conventional transmitter, thus increasing power consumption. One of approaches to solving the problems is a direct-conversion architecture that directly perform a frequency up-conversion without IF conversion.
The conventional direct-conversion, however, must generate a quadrature signal in a local oscillator. The phase of an RF signal must be accurately shifted by 90 degrees. Therefore, hardware architecture becomes more complicated and much power is consumed. Moreover, the modulation quality of the transmitter is degraded.